This application claims the priority of Germany Application No. 197 03 796.8, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a sensor for selective detection of ammonia (NH.sub.3) in NO.sub.x -containing high-oxygen gases. Such gases include exhaust gases of diesel engines, exhaust gases of injected Otto engines or the exhaust of a power plant.
It is known that nitrogen removal from diesel engine exhaust gases can be carried out by selective catalytic reduction (SCR) using ammonia as a reducing agent. Ammonia can be carried in vehicles either directly or in a compound.
For the catalytic reaction, ammonia is metered to engine exhaust gas at a fixed ratio to NO.sub.x. The NH.sub.3 --NO.sub.x ratio for achieving maximally possible NO.sub.x -conversion would have to be exactly 1. A smaller ratio leads to a smaller conversion; a larger ratio leads to an "NH.sub.3 breakthrough". Since neither NO.sub.x sensors nor NH.sub.3 sensors suitable for the vehicle are available, NO.sub.x values are obtained from characteristic engine diagrams stored in a computer.
NH.sub.3 emission must be securely prevented in all operating conditions. However, characteristic NO.sub.x diagrams do not apply to individual engines, but to a line of products, so that fluctuations in NO.sub.x crude gas content occur caused by manufacturing, while characteristic engine diagram points are the same. In addition, a momentary catalyst condition (temperature, NO.sub.x and NH.sub.3 charge) may be different while characteristic diagram points are the same. Accordingly, an NH.sub.3 /NO.sub.x ratio is selected in practice which is clearly smaller than 1, (for example, 0.6) so that a sufficiently large safety margin can be maintained. Thus, poorer nitrogen oxides removal is intentionally accepted.
In order to achieve NO.sub.x limit values, NO crude engine emission must be reduced further than necessary. With respect to diesel engines, this reduces efficiency.
A better use of a nitrogen oxide removal catalyst could be achieved with an NH.sub.3 sensor. Such a sensor could be mounted as a control element or as an NH.sub.3 breakthrough sensor. Under diesel engine exhaust gas conditions, the NH.sub.3 sensor would have to detect a small amount of NH.sub.3 in a secure manner without any significant cross sensitivity to other relevant gases (mainly NO.sub.x, H.sub.2 O and O.sub.2).
European Patent Document EP 0426 989 B1 describes a chemical sensor for gases on the basis of zeolite-coated, directly heated, planar interdigital capacitors (in the following abbreviated as "IDK"). In one embodiment, this sensor consists of a platinum-containing zeolitic layer.
During the investigations concerning NH.sub.3 sensitivity, it was found that IDK sensors which are coated with precious-metal-containing zeolites cannot be used as NH.sub.3 sensors in exhaust gas for two reasons: either they lack sufficiently high NH.sub.3 sensitivity (FIG. 1), or they have NH.sub.3 sensitivity, (FIG. 2a) but simultaneously have a very high cross sensitivity to NO (FIG. 2b).
FIG. 1 is a representation of impedance spectra (frequency range: 20 Hz to 1 MHz) of an IDK sensor of the prior art, which is coated with PtZSM5 and acted upon by 1,000 ppm NH.sub.3 (triangular it symbols) and 0 ppm NH.sub.3 (rectangular symbols) in 10% O.sub.2, 5% abs H.sub.2 O, with "abs" representing percent by volume.
FIGS. 2a and 2b are representations of impedance spectra of an IDK sensor of the prior art, which is coated with a PtY zeolite and in 10% O.sub.2, 5% abs H.sub.2 O; FIG. 2a: at 1,000 ppm NH.sub.3 (triangular symbols) and 0 ppm NH.sub.3 (rectangular symbols); FIG. 2b: at 1,000 ppm NO (triangular symbols) and 0 ppm NO rectangular symbols). FIG. 2b shows the impedance spectra of this IDK sensor with and without NO. Here, it is found that this type of sensor is also sensitive to NO. This cross sensitivity to NO does not permit a use as an NH.sub.3 sensor in exhaust gas.
It is an object of the invention to provide a sensor which detects small amounts of NH.sub.3 in a secure manner and without any significant cross sensitivity to other relevant gases (mainly NO.sub.x, H.sub.2 O and O.sub.2).
This object is achieved according to the invention with an NH.sub.3 sensor comprising a component part acting as a capacitor and a gas-permeable sensitive layer as the dielectric, the sensitive layer being a hydrophobic, precious-metal-free zeolite of a low acidity (acidic strength) which has an ordered crystalline structure of primary pores whose diameter is in the order of the gas-kinetic diameter of NH.sub.3.
The interaction of the molecule to be detected with the precious-metal-free zeolitic solid state sensor according to the invention takes place completely differently than in the case of the sensor described in European Patent Document EP 0 426 989 B1. In the case of the platinum-containing zeolitic layer, a catalytic conversion of the gas molecules takes place at the platinum clusters within the zeolitic pore system. By means of the reaction, which does not take place infinitely fast with respect to time, the mobility of the cations (generally sodium ions) of the zeolite, is hindered, but they can be moved at a raised temperature through the electric alternating field. The reduction of the mobility of cations becomes noticeable at low frequencies as a change of impedance.
In the case of sensors according to the invention having a precious-metal-free hydrophobic zeolite layer as the ammonia-sensitive layer, no catalytic conversion of the NH.sub.3 takes place, because the eligible zeolites have no catalytic conversion activity for NH.sub.3. In contrast, the adsorption of the ammonia molecule plays a determining role with respect to the behavior of the sensor. The protons of the adsorbed ammonia molecules are the charge carriers whose mobility in the electric alternating field generates the sensor signal. Ionic conductivity changes in the whole examined frequency range from 20 Hz to 1 MHz. At the high frequencies, only the protons can still follow the alternating field; the much heavier other eligible cations can no longer do so.
The hydrophobic character of the zeolites is determined by their tendency to absorb polar molecules (such as water) only in very low quantities, but homopolar compounds (such as hydrocarbons) in large quantities. Since, during the generating of the signal of the NH.sub.3 sensor according to the invention, the adsorption condition of the NH.sub.3 molecule in the zeolitic pore system plays the decisive role, it is important that only a small quantity of water is present in the pore system. On the one hand, the NH.sub.3 could dissolve in the pore water as ammonium hydroxide (NH.sub.4 OH) and could partially dissociate. On the other hand, the cross sensitivity to the water would be too high.